Spectrum geophysics

Services: Vibration Monitoring

Vibration monitoring methods are surface methods that use the properties of acoustic waves to measure the intensity of ground vibrations caused by these waves. Traditional reasons for vibration monitoring are 1) complaints from residents that new wall, floor or ceiling cracks have been caused by nearby blasting or pile driving activities; and 2) the need for contractors to document that established threshold levels of vibration and sound have not been exceeded during blasting or pile driving activities. Recently, it has become useful to measure vibrations for monitoring the effects of traffic to nearby residents. Two parameters are commonly measured by Spectrum to monitor blast-induced or construction-induced vibrations at a site: particle velocity and air-blast overpressure (sound). The parameter recorded depends somewhat on the reason for the monitoring and the primary concerns for the project; however, particle velocity is almost always measured regardless of the purpose for monitoring.





• Particle Velocity

  
  In essence, particle velocity is related to the vibration you “feel” when standing near a blasting or pile-driving site. It is a measure of the intensity of the particle motion of blast- or construction-induced seismic waves, and is somewhat analogous to the Richter Magnitude of earthquakes. Particle velocity has long been the measured parameter for blast monitoring, but more recently it has been recorded for construction-induced vibrations. Because this method is similar to other seismic methods in that it uses a geophone and a seismograph to make the measurements, the equipment has evolved with advances in seismic and computer technologies. As a result, the equipment for measuring particle velocity is easily deployed in the field, can be programmed to record data automatically, and can store large amounts of data seamlessly.

  
  Spectrum utilizes Instantel’s BlastMate III or MiniMate Plus seismographs along with a three-component geophone and associated cabling to measure and record particle velocities. During monitoring, a three-component geophone is established at the ground surface and used to measure frequency-based particle velocities in three mutually-perpendicular directions for a particular time interval. In one popular recording mode, particle velocities and their frequencies are monitored during a specified time “listening” interval (1 minute, for example) but just the peak particle velocity for each component that occurred during that time interval is recorded and saved by the seismograph. The monitoring then continues all day (or all night) long and, for each component, the data record contains the peak particle velocity measured each minute for the entire monitoring period. Because these recorded particle velocities come with a time stamp, peak values can actually be correlated with field activities and determinations can be made regarding sources of vibration. Recorded particle velocities and their associated frequencies are then graphed and compared with known threshold values of frequency-based particle velocity that cause cracking or other types of damage to structures.
  

  
  
  
• Overpressure

  Overpressure refers to the amplitude of sound pressure waves above atmospheric pressure within a specific bandwidth (generally between 1 Hz and 100 Hz). Overpressure is distinctly different from noise because it includes the low-frequency (between 1 Hz and 20 Hz) portion of sound pressure waves termed air blast that is the most damaging to structures but is not audible to humans; whereas noise refers to that portion of air pressure waves that can actually be heard by humans (roughly in the frequency band of 20 Hz to 20 kHz). Ironically, the noise portion of sound pressure waves is much less damaging to structures but commonly causes more annoyance to nearby residents than air blast does. Similar to particle velocity, the overpressure value is a measure of the intensity of blast or construction-induced sound pressure waves.

  Spectrum utilizes Instantel’s BlastMate III or MiniMate Plus seismographs along with a linear microphone and associated cabling to measure and record overpressures. During monitoring, a linear microphone is established adjacent to the seismograph such that it is pointing toward the sound source. This microphone is then used to measure frequency-based air blast and sound overpressures for a particular time interval. Like particle velocity, overpressures and their frequencies can be monitored during a specified time “listening” interval (1 minute, for example) where the peak overpressure that occurred during that time interval is recorded and saved by the seismograph. The monitoring then continues all day (or all night) long and the data record contains the peak overpressure measured each minute for the entire monitoring period. Because overpressures come with a time stamp, peak values can actually be correlated with field activities and determinations can made regarding sources of air blast and noise. Recorded overpressures and their associated frequencies are then graphed and compared with known threshold values of frequency-based overpressure that cause cracking or other types of damage to structures.

  
  
  
METHODS IN ACTION


Vibration Monitoring of Pile Vibrating – SANTA BARBARA COUNTY, CA

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  Vibration and sound monitoring are useful methods for documenting and controlling the effects of blasting, pile driving and other construction activities on man-made structures. Measured values can be compared with established threshold levels for structural damage and used to prevent generated vibrations and overpressures from exceeding safe levels.
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  Vibration monitoring was conducted in Santa Barbara County, California to document the ground vibrations and air-blast overpressures generated during pile vibrating activities, as there was concern regarding claims of structural damage from nearby residents. These measurements were then referenced to the frequency-based threshold levels for structural damage established by the US Bureau of Mines (USBM) and the Office of Surface Mining and Reclamation (OSMRE).
• Spectrum established two stations at the site and monitored the vibrations and overpressures generated by pile-vibrating over the course of 4 days. A Blastmate III monitor coupled to a three-component geophone and a linear microphone were used to continuously record peak particle velocities and peak overpressures (and their associated frequencies) at one-minute intervals for the duration of pile vibrating activities. In addition, background vibrations and overpressures were monitored in order to compare the daily vibration and overpressure levels from significant air and vehicle traffic with the levels associated with pile vibrating. Field logging of pile vibrating activities was used to compare peak particle velocities and overpressures with specific activities, and waveforms of pile vibrating vibrations were recorded in order to identify dominant frequencies associated with pile driving. Data were processed using the Blastware® software package. Peak particle velocities for all three components were found to fall well below the established frequency-based threshold levels for structural damage.